U.S. patent number 4,118,734 [Application Number 05/737,321] was granted by the patent office on 1978-10-03 for optical videodisc with variable width tracks.
This patent grant is currently assigned to U.S. Philips Corporation. Invention is credited to Gijsbertus Bouwhuis, Josephus Johannes Maria Braat, Bernardus Antonius Johannus Jacobs.
United States Patent |
4,118,734 |
Bouwhuis , et al. |
October 3, 1978 |
Optical videodisc with variable width tracks
Abstract
A disc-shaped record carrier is described on which information
is recorded in an optical structure of trackwise arranged areas and
intermediate areas. By adapting the average dimensions of the areas
to the average spatial frequencies on the record carrier a read
signal can be obtained of sufficient modulation depth and minimal
distortion. This is accomplished by increasing the track width with
decreasing radius of the record carrier.
Inventors: |
Bouwhuis; Gijsbertus
(Eindhoven, NL), Braat; Josephus Johannes Maria
(Eindhoven, NL), Jacobs; Bernardus Antonius Johannus
(Eindhoven, NL) |
Assignee: |
U.S. Philips Corporation (New
York, NY)
|
Family
ID: |
23756650 |
Appl.
No.: |
05/737,321 |
Filed: |
October 29, 1976 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
442397 |
Feb 14, 1974 |
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Current U.S.
Class: |
369/111; 347/262;
369/117; 369/275.3; 720/718; G9B/7.01; G9B/7.039; G9B/7.102 |
Current CPC
Class: |
G11B
7/0045 (20130101); G11B 7/24085 (20130101); G11B
2007/13727 (20130101) |
Current International
Class: |
G11B
7/00 (20060101); G11B 7/125 (20060101); G11B
7/013 (20060101); G11B 7/0045 (20060101); H04N
005/76 () |
Field of
Search: |
;179/1.4C,1.4R,100.41,1.3V,1.3R,1.3F,1.3N ;358/128,127,130,132
;274/41.6S,41.6R ;346/76L ;235/61.12R ;340/173LM |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Konick; Bernard
Assistant Examiner: Faber; Alan
Attorney, Agent or Firm: Trifari; Frank R. Cohen; Simon
L.
Parent Case Text
This is a continuation of application Ser. No. 442,397, filed Feb.
14, 1974, now abandoned.
Claims
What is claimed is:
1. A record carrier of the type on which information is recorded in
the form of an optical structure on substantially concentric tracks
in the form of regions and intermediate areas, said regions having
different optical properties than the intermediate areas and the
lands between the tracks, the improvement wherein the width of the
tracks increases as the radial positions of the tracks
decrease.
2. Apparatus for writing information on a record carrier in the
form of an optical structure of substantially concentric tracks as
regions and intermediate areas, the regions having different
optical properties than the intermediate areas and the lands
between the tracks, the apparatus being of the type comprising a
radiation source for supplying the writing beam of radiation, and a
modulation device means for varying the pulse repition rate of
pulses of said radiation in accordance with the information to be
written, the improvement wherein the apparatus comprises
compensating means for widening the beam independently of the
information to be written as the radial position of the tracks
decreases.
Description
The invention relates to a record carrier on which information is
recorded in an optical structure of trackwise arranged areas and
intermediate areas, which areas have a different influence on a
read beam of radiation than the intermediate areas and the lands
between the tracks, the lengths of the individual areas and
intermediate areas defining the information, for which record
carrier the average spatial frequency of the areas in the read
apparatus varies. The invention also relates to an apparatus for
writing information on such a record carrier.
In this respect tracks is to be understood to mean a multitude of
concentric tracks or, in the case of a round record carrier, one
continuous spiral track, which spiral track may be divided into a
number of quasi-concentric tracks. The average spatial frequency of
the areas is to be understood to mean the average number of areas
per unit of length.
For reading a record carrier provided with an optical information
structure it has been proposed in U.S. Ser. No. 344,866, filed Mar.
26, 1973 abandoned in favor of U.S. Ser. No. 536,161 filed Dec. 24,
1974, now U.S. Pat. No. 3,956,582 issued May 11, 1976, to project a
radiation spot of dimensions greater than the width of a track onto
the information structure. The radiation emerging from the record
carrier is concentrated onto a radiation sensitive detector by a
lens of limited numerical aperture. At the areas in the information
structure diffraction occurs, the diffracted beams of radiation for
the most part falling outside the lens aperture. Consequently, if
an area is located in front of the lens, less radiation will reach
the detector than if there is no area in front of the lens.
In order to obtain an optimum optical signal the geometry of the
areas on the record carrier and the dimensions of the radiation
spot, as well as the intensity distribution over the radiation spot
should match each other. The optimum aperture of the lens which
forms the radiation spot is then defined for a specific geometry of
the areas. The average spatial frequency and thus the average
length of the areas, however, may vary over the various tracks.
Therefore, all the tracks cannot be read in an optimum manner by
means of a single radiation spot formed by one specific lens.
The conversion of the information recorded in the optical structure
of the record carrier into an electrical signal need not always be
linear. It may happen that the detector receives a certain
threshold intensity, whilst the edge of the area is not yet
correctly positioned relative to the radiation spot. This results
in a distortion of the detector signal, especially at low spatial
frequencies, i.e. at great average lengths of the areas and the
intermediate areas in the tracks.
It is an object of the invention to provide a record carrier which
solves one or both of these problems. For this purpose, according
to the invention, the average dimensions of the areas in the tracks
correspond to the average spatial frequency in the tracks in such a
way that, when reading the record carrier by means of a read beam
of radiation which is focussed onto the information structure to a
radiation spot larger than the track width and, in the case of
several tracks, is smaller than the sum of the track width, and
twice the distance between two consecutive tracks, a signal of
sufficient modulation depth and minimal distortion is obtained.
According to a further feature of a record carrier according to the
invention, the track width increases as the average spatial
frequency of the block in the tracks increases. This allows the
record carrier according to the invention to be read in an optimum
manner by means of a single radiation spot of one specific geometry
and one specific intensity distribution over the radiation spot.
Adaptation of the track width furthermore ensures that those
positions of the radiation spot and areas relative to each other,
where the intensity of the radiation to the detector is half the
difference between the maximum and minimum intensities of the
radiation to the detector, sufficiently approximate the desired
positions.
According to a further feature of a record carrier, according to
the invention, this may also be achieved for a low average spatial
frequency of the areas by making the average length of the areas
smaller than one half the period which would correspond to the
intended spatial frequency.
According to the invention yet another possibility at a low average
spatial frequency is to taper the ends of the areas off to a point
in the longitudinal direction of the tracks.
The invention will now be described in more detail with reference
to the drawing, in which:
FIG. 1 shows a part of the optical structure of a record carrier to
be read,
FIG. 2 represents a previously proposed apparatus for reading such
a record carrier,
FIG. 3 represents the amplitude distribution of a radiation spot
before and after interaction with an area of the record
carrier,
FIG. 4 shows parts of two information tracks of different spatial
frequencies,
FIG. 5 represents the signals produced by interaction of a
radiation spot with the tracks of FIG. 4,
FIG. 6 shows a part of a track of high spatial frequency adapted
according to the invention,
FIG. 7 shows the intensity variation of a radiation beam emerging
from the record carrier as a function of the position of the edge
of an area relative to the radiation spot,
FIG. 8 represents a deformation of the detector signal which occurs
at low spatial frequencies,
FIGS. 9 and 10 show parts of optical information structures which
has been corrected for said deformation in accordance with the
invention,
FIG. 11 shows an apparatus for writing information on a record
carrier according to the invention.
FIG. 12 shows a part of the optical structure of a record carrier
according to the invention.
FIG. 1 represents a part of an optical structure of a record
carrier to be read, specifically a round record carrier 1. On the
record carrier 1 a number of regions g are arranged in tracks 2.
The blocks have a different effect on a rediation beam which is
incident on the record carrier than the rest of the record carrier,
i.e. than the intermediate areas t and the information-free lands
3. The tracks may be arranged parallel to each other, i.e.
concentric relative to the center of the record carrier. The record
carrier may also be provided with a continuous spiral track. The
lengths of the areas and the intermediate areas are determined by
the information stored in the track.
A radiation beam, not shown, produces a radiation spot V on the
optical structure. By moving the record carrier in the direction
indicated by the arrow 4, the radiation beam is time-modulated in
accordance with the sequence of regions and intermediate areas in a
track. The diameter d of the radiation spot V is greater than the
width of the tracks 2, but smaller than the sum of the track width
and twice the width of the information-free lands 3.
The numerical aperture of a lens L (see FIG. 2), which serves to
image the record carrier onto a detector D, is selected so, that
said lens cannot image the narrow regions g. Then, as long as a
radiation spot is projected outside an region g, the detector D
will receive a maximum amount of radiation. When a radiation spot
is projected onto an region g of a finer structure than d much
radiation is diffracted outside the entrance pupil of the lens L,
so that substantially no radiation impinges on the detector.
The information may be stored in the record carrier as an amplitude
structure. The regions and areas being coplanar with the surface of
the record carrier. The regions may then be radiation-absorbing,
while the record carrier itself may be radiation-transmitting or
radiation-reflecting, or conversely, the regions may be
radiation-transmitting or radiation-reflecting, while the record
carrier is radiation-absorbing. The plane of the regions may also
be disposed at a small distance from the surface of the record
carrier. An example of such a phase structure is a reflecting
record carrier, i.e. which pits are pressed at the location of the
areas.
For a maximum modulation of the detector current by said pattern of
pits on the record carrier the radiation beams reflected at the
bottom of a pit and at the record carrier surface adjacent thereto
must have a difference in phase of 180.degree. and have the same
intensity. The 180.degree. phase difference is obtained by giving
the pit a depth equal to a quarter of the wavelength of the
radiation used, so that the difference in path length between the
two reflections will be half a wavelength. Equality of the
intensities is obtained by ensuring that an equal amount of
radiation is incident beside and in a pit. Thus, every track
geometry is associated with an optimum intensity distribution over
the radiation spot and, because that distribution depends on the
diffraction at the lens aperture, also with an optimum value of the
aperture of the lens which forms the radiation spot.
This is illustrated in FIG. 3. The curve a represents the amplitude
A as a function of the radius r in a round radiation spot. If said
radiation spot is projected onto a pit k which is situated in a
record carrier at a depth of a quarter wavelength an amplitude
distribution is obtained in accordance with the curve c. The
radiation reflected by the bottom of the pit is 180.degree. phase
shifted relative to the radiation reflected by the area surrounding
the pit. The detector is located on the optical axis in a point
which is situated comparatively far from the pit. The total
radiation in this point can be determined by addition of the
complex amplitudes of all components e and f. It is evident that by
an appropriate choice of the amplitude distribution and of the
dimensions of the pit the sum of the complex amplitudes and thus
the intensity of the radiation which is incident on the detector,
can be made negligibly small.
As stated previously, the length of the pits is determined by the
information which is recorded in the record carrier. For a great
number of consecutive pits, however, an average length can be
given. Said average length may now vary over the record carrier.
This is for example the case with a round disc-shaped record
carrier on which a television programme is written and of which
each concentric or quasi-concentric track contains the information
of one frame. It is obvious that for such a record carrier the
average length of the pits in the outer track is greater than that
of the pits in the inner track, for example by a factor 3 for an
inner diameter of 10 cm and an outer diameter of 30 cm. This means
that when the aperture of the lens L is selected so that an outer
track is read in an optimum manner, an inner track cannot be read
in an optimum way by means of said lens.
FIG. 4 represents parts of an information structure with a great
average pit length (l) and an information structure with small
average pit length (l'). In reality the lengths of the pits in a
track may still be modulated in accordance with the information
stored in the track.
It is assumed that the amplitude distribution over the radiation
spot and the dimensions of the radiation spot are such that the
track of an average pit length l allows optimum reading. This means
that when the radiation spot is projected on a pit the sum of the
complex amplitudes of the reflected radiation components j and h is
substantially zero, so that the detector receives a minimum amount
of radiation. If the radiation spot is projected midway between two
pits, no diffraction will occur, so that the amount of radiation on
the detector is maximum. The variation of the radiation intensity
as a function of the travel x of the read spot in the direction of
the track is then as represented by the curve u in FIG. 5.
If the same radiation spot is used for reading the information
track of short average pit length l', the variation of the
intensity of the radiation intercepted by the detector will be far
more unfavourable as is represented by the curve u' in FIG. 5. The
spacing between the pits in the longitudinal direction of the track
is smaller than the diameter of the radiation spot, so that the
radiation spot is always projected on a pit or on parts of adjacent
pits. As a result, the maximum radiation intensity at the detector
will be substantially smaller than when reading a track of great
average pit length. When the radiation spot is projected onto one
pit the complex amplitudes of the reflected radiation components j
and h will be anything but equal, so that the minimum of the
radiation intensity which is intercepted by the detector is higher
than when a track of great average pit length l is read. The
modulation depth of the detector signal when reading a track of
small average pit length is smaller than when reading a track of
great average pit length.
According to the invention the situation can be improved upon for a
track of small average pit length, allowing for the amplitude
distribution over the radiation spot, by adapting the widths of the
pits in such a way that when the radiation spot is projected onto a
pit, the intensities of the radiation reflected by the bottom of
the pit and of the radiation reflected by the area surrounding the
pit approximate each other closely enough.
FIG. 6 shows a part of an information track of an average length
pit l', according to the invention. The width of the pits is
substantially greater than in FIG. 4. This ensures that the level
of the minimum radiation intensity at the detector is reduced, so
that the modulation depth of the detector signal becomes greater
than that according to the curve u' in FIG. 5.
However, widening the tracks also has an influence on the d.c.
component in the detector signal. It is obvious that the d.c.
component varies with a variation of the average spatial frequency
i.e. such that at increasing average spatial frequency of the pits
the d.c. component decreases; compare levels n and n' in FIG. 5.
The variation of the d.c. component increases over a certain range
of spatial frequencies as the pits become wider. The variation of
the d.c. component during reading of a record carrier may present
difficulties when electronically processing the electrical signal
from the detector. If a minimal variation of the d.c. component is
desirable, for example for a certain coding of a television signal
on the record carrier, the track width will be the result of a
compromise. In that case some of the improvement in modulation
depth attainable according to the invention will have to be
sacrificed in favour of an as constant as possible d.c. component
in the detector signal.
In additional to being useful in a round record carrier with an
equal amount of time information per track, the invention may also
be employed in any record carrier in which for some reason the
spatial information density in the optical structure changes in a
specific direction.
It is assumed that when the center of the radiation spot passes the
beginning or end of a pit, while the record carrier is being read,
the radiation intensity intercepted by the detector amounts to half
the difference between the possible maximum and minimum radiation
intensity at the detector. However, in practice this requirement is
not always met and non-linearities result, especially at low
average spatial frequencies.
FIG. 7 shows how, in the case of a pit structure of low spatial
frequency, the intensity of the radiation to the detector varies as
a function of the position of the radiation spot relative to the
pit. In this case it is assumed that a radiation spot s formed by
Gaussian intensity distribution has a diameter of 1.3 .mu.um. The
pit has a length of 3 .mu.m and is 0.8 .mu.m wide. The consecutive
positions of the end of the pit relative to the radiation spot are
shown dotted and are designated by 10, 11 and 12. The intensity
I.sub.o is half I.sub.max - I.sub.min, I.sub.max representing the
total intensity of the radiation to the detector if the radiation
spot is projected outside a pit I.sub.min is the minimum intensity
of the radiation directed towards the detector if the radiation
spot is projected onto a pit. The "hump" in the curve which
represents the intensity in FIG. 7 is caused by the fact that the
pit and the radiation spot are not adapted to each other. For
narrower pits the "hump" will disappear. The Figure shows that
already in situation 11, i.e. when the end of the pit is still at a
comparatively great distance from the center m of the radiation
spot, the intensity of the radiation reflected to the detector has
already dropped to about 1/2 I.sub.o. As a result of this, the
electrical signal supplied by the detector will have a shape as
represented by the uninterrupted line in FIG. 8.
The signal is strongly distorted relative to the signal which is
represented by a dotted line, which would be obtained if the value
1/2 I.sub.o were attained if the end of the pit should coincide
with the center m of the radiation spot s.
According to the invention the said non-linearity can be
substantially avoided, as shown in FIG. 12, by making the outer
tracks of the said round record carrier, narrow. This ensures that
the point in the radiation spot where the end of a pit must be
located in order that the radiation intensity to the detector
attains the value 1/2 I.sub.o is shifted towards the center of the
radiation spot. A higher spatial frequencies, when the radiation
spot is continually projected onto a pit or parts of adjacent pits,
the said non-linearity is no longer significant. The tracks where
the average pits are shorter may be wider, so that the requirement
for optimum reading is met: as many as possible equal intensities
of the radiation emerging from a pit and of that from the area
surrounding a pit. Thus, the influence of two errors can largely be
eliminated by a single step, namely by making the tracks with a low
spatial frequency of the optical information narrower than the
tracks with a high spatial frequency.
According to the invention it is equally possible in the case of a
record carrier with constant track width to take a step so as to
prevent non-linear reading. The pits themselves can be made
slightly shorter. This is illustrated in FIG. 9 by dotted lines;
the uninterrupted lines represent a previously proposed pit
structure. By making the average lengths of the pits smaller than
half the average period of the pit spatial frequency it can be
achieved that the spacings between the positions at which the
reflected radiation intensity attains the level 1/2 I.sub.o become
equal to the desired spacing of half a period.
The same effect is obtained by tapering the pits to a point as is
shown in FIG. 10.
The invention is described with reference to a record carrier
having a reflecting pattern of pits. However, because the invention
in general yields an improvement for a record carrier which is
destined to be read in accordance with the principle: radiation
spot greater than the track width, it may be employed in many other
record carriers. For example, a fully transparent record carrier is
conceivable with areas disposed at half a wavelength from the
record carrier surface (phase structure) or a transparent or
reflecting record carrier provided with trackwise arranged
radiation-absorbing areas (amplitude structure). The advantages of
the read principle: radiation spot greater than the track width,
however, are particularly predominant for a phase structure,
because a small phase variation results in a substantial variation
of the detector signal.
For writing information on a record carrier according to the
invention use can be made of a special write apparatus. Said
apparatus is based on a previously proposed device (described in
the previous U.S. Pat. No. 3,894,179 and is provided with special
means according to the invention for influencing the write beam of
radiation, independently of the information to be recorded, in such
a way that in the eventual record carrier the average dimensions of
the areas match the average spatial frequencies on the record
carrier. The eventual record carrier may be the record carrier on
which information is written with the aid of the said apparatus,
called the master record carrier, or a record carrier which via a
pressing method is obtained from the master record carrier.
As is shown in FIG. 11, the apparatus includes a radiation source
18, which supplies a radiation beam of sufficient power. Said beam
is directed to the record carrier 1 on which information is to be
written via the prisms 19, 20 and 21 and concentrated to a small
radiation spot by an objective lens 27. The record carrier is
provided with a layer which is sensitive to the radiation used. The
radiation path between the source 18 and the record carrier 1
furthermore includes a diaphragm 22 and an electro-optic radiation
modulator 23. Said radiation modulator in conjunction with the
electronic control device 24 constitutes a modulation unit. By
means of said modulation unit the information, for example a
television programme, which is applied to the terminals 30, 31 in
the form of an electrical signal, can be converted into radiation
pulses of the laser source. At specific instants, given by the
information at the terminals 30, 31 radiation spots are projected
on the record carrier.
The record carrier has a round circumference and is rotated about
its axis with the aid of a motor 15, which by means of a carriage
16 is radially movable, thus allowing for example a spiral track to
be written on the record carrier 1.
In order to enable the width of the tracks to be written to be
varied in accordance with the average spatial frequency (compare
FIGS. 4 and 6), care is taken that according to the invention the
size of the radiation spot projected onto the record carrier is
adjustable in one direction. For this purpose, for example
diaphragm 22 may be adjustable in one direction. However,
preferably a diaphragm which is adjustable in one direction is
disposed in the immediate vicinity of the objective lens 27, as is
designated by 32.
According to the invention, the areas with tapered ends as shown in
FIG. 10 can be written by adapting the rise times of the radiation
pulses to be formed in the modulation device (23, 24). This can be
achieved, for example, by including a variable capacitance 33 in
parallel with the capacitance of the electro-optic modulator, so
that the RC-time, which determines the rise time, of the modulation
device can be adjusted to different values for different spatial
frequencies on the record carrier.
The modulation device (23, 24) also enables a pattern of pits as
shown dotted in FIG. 9 to be obtained by electronic means. This is
for example possible, independent of the information to be written,
by inserting a variable delay between the leading and trailing
edges of the square-wave information signals applied to the
terminals 30 and 31.
As indicated in the aforementioned U.S. Pat. No. 3,956,582 the
record carrier may be utilized to record color television signals.
Such signals are modulated by picture information in such a manner
that, while the instantaneous frequency of the signal constantly
changes the signals vary about an average value to which a normal
television tuner is set. Thus an average television signal can be
said to have an average temporal frequency, or frequency with
respect to time. When such a signal is recorded along the length of
a track on a disc or along the length of a television tape the
temporal frequency of the signal is registered as a spatial
frequency on the recording medium. The spatial frequency of the
recorded data depends on the temporal frequency of the signal to be
recorded and on the speed between the recording medium and the
recording apparatus or stylus. As with the temporal frequencies the
spatial frequencies will also vary within a small section of
recorded track, but will vary about an average spatial frequency on
a relatively long track section if the recording medium-stylus
speed is kept constant. On a reel of magnetic television recording
tape, therefore, the average spatial frequency throughout the reel
of tape corresponds to the average temporal frequency of the
television channel recorded. Recording on a disc, however, involves
changes in the average spatial frequency due to the change in
recording medium-stylus speed. Thus, on a twelve inch record having
an inner recording track diameter of 3 inches, the change in
recording medium-stylus speed is 400%. That is, the recording
stylus speed at the twelve inch diameter is four times that at the
three inch diameter. This produces a shift in the average spatial
frequency measured over each revolution. In view of the fact that
this produces disturbing reactions in the read beam when the disc
is played back the disc is provided with a track that progressively
widens as it nears the center of the disc. As mentioned above, this
may be accomplished using an adjustable diaphragm, the adjustment
of which may be made manually, or, if a diaphragm of the type used
in simple automatically adjusting cameras is used, the adjustment
may be made electrically in response to any electrical pick up
connected to the apparatus for moving the beam. Such pick ups are
commonly used in most automatic machine tools.
* * * * *